If the Supreme Court’s more liberal justices seemed reluctant Wednesday to find that a key provision of the Voting Rights Act was no longer needed, its more conservative justices seemed just as skeptical that the formula used to determine the statute’s geographic reach is still valid.

The court’s decision in Shelby County v. Holder could dramatically alter the scope of the landmark 1965 civil rights law. Shelby County, Ala., has questioned the constitutionality of a portion of the statute that determines which areas, mainly in the South, must ask the federal government before changing voting procedures.

During a charged oral argument that extended past its allotted hour, the justices considered the county’s case and, in doing so, the process by which Congress in 2006 reauthorized the preclearance provisions for another 25 years. A ruling is expected before the court recesses at the end of June.

“There is no question that the Voting Rights Act has done enormous good,” Justice Samuel A. Alito Jr. said. “But when Congress decided to reauthorize it in 2006, why wasn’t it incumbent on Congress ... to make a new determination of coverage? Maybe the whole country should be covered. Or maybe certain parts of the country should be covered based on a formula that is grounded in up-to-date statistics.”

Shelby County and other covered jurisdictions have noted that the coverage formula still uses evidence of tactics used to limit voting and lower turnout during elections in the 1960s and 1970s; Congress opted to not tinker with that formula during the 2006 reauthorization.

The Voting Rights Act was part of President Lyndon B. Johnson’s Great Society programs. Its provisions, including two that uniquely singled out areas with a history of discrimination for pre-emptive enforcement, were intended to ameliorate a century of race-based voting discrimination that lingered after other laws failed to make meaningful inroads.

Section 4 of the statute contains a formula that determines what areas have a history of discriminating against minority voters. Section 5 requires those jurisdictions to clear any changes to their voting procedures with the Justice Department or a federal court. Both require periodic reauthorization by Congress.

Justice Antonin Scalia suggested that lawmakers had little incentive to change or vote against a politically sensitive law, noting the increasingly comfortable margins by which Congress renewed the preclearance portions of the statute in 1970, 1975, 1982 and 2006.

“I think it is attributable, very likely attributable, to a phenomenon that is called perpetuation of racial entitlement,” Scalia said.

“Even the name of it is wonderful: the Voting Rights Act. Who is going to vote against that?” he asked pointedly.

Solicitor General Donald B. Verrilli, who presented the government’s case defending the law, called Section 5 the “principal engine” driving the progress that has been made since the statute was passed.

Justices Sonia Sotomayor, Stephen G. Breyer, Ruth Bader Ginsburg and Elena Kagan were just as probing when questioning Bert Rein, who represented Shelby County. The justices asked why, if covered areas continue to show evidence of discrimination, there was any evidence that the current formula was outdated.

“Assuming I accept your premise, and there’s some question about that, that some portions of the South have changed, your county pretty much hasn’t,” Sotomayor interjected at the beginning of Rein’s argument. “Why would we vote in favor of a county whose record is the epitome of what caused the passage of this law to start with?”

“I don’t know what they’re thinking exactly,” Breyer said of the most recent congressional reauthorization. “But it seems to me one might reasonably think this: It’s an old disease, it’s gotten a lot better, a lot better, but it’s still there. So if you had a remedy that really helped it work, but it wasn’t totally over, wouldn’t you keep that remedy?”

Though Justice Anthony M. Kennedy, thought to be the likely swing vote on the case, seemed open to subjecting areas with a history of voting discrimination to pre-emptive enforcement, he questioned whether the statute is appropriate as currently applied. The Marshall Plan and the Northwest Ordinance were once-needed laws that were later abandoned, he pointed out.

“Times change,” Kennedy said. “Congress just didn’t have the time or energy to do this, it just re-enacted it.”

Congressional Democrats defended the law during a Wednesday news conference on the court’s steps.

“The Voting Rights Act restored justice, equality and fairness to our country’s most sacred right: the right to vote. We are now witnessing unprecedented attacks on the right to vote and now more than ever, we must also fight to maintain its legacy and integrity,” said Congressional Hispanic Caucus Chairman Rubén Hinojosa of Texas.